Moving laser beam readers or laser scanners, as well as solid-state imaging systems or imaging readers, have been used to electro-optically read targets, such as one-dimensional bar code symbols, particularly of the Universal Product Code (UPC) type, each having a row of bars and spaces spaced apart along one direction; two-dimensional symbols, such as Code 49 that introduced the concept of vertically stacking a plurality of rows of bar and space patterns in a single symbol as described in U.S. Pat. No. 4,794,239, and PDF417 that increased the amount of data represented or stored on a given amount of surface area as described in U.S. Pat. No. 5,304,786; and non-symbol targets, such as documents, signatures and receipts.
Moving laser beam readers generally include a laser for emitting a laser beam, a focusing lens assembly for focusing the laser beam to form a beam spot having a certain size at a focal plane in a range of working distances, a scan component for repetitively scanning the beam spot across a target symbol in a scan pattern, for example, a scan line or a series of scan lines, across the symbol multiple times per second, e.g., forty times per second, a photodetector for detecting laser light reflected and/or scattered from the symbol and for converting the detected laser light into an analog electrical signal, and signal processing circuitry including a digitizer for digitizing the analog signal, and a microprocessor for decoding the digitized signal based upon a specific symbology used for the symbol.
The imaging reader includes a solid-state imager or sensor having an array of cells or photosensors that correspond to image elements or pixels in a field of view of the imager, an aiming light assembly having an aiming light source, e.g., an aiming laser, and an aiming lens for generating an aiming light pattern or mark on a target prior to reading, an illuminating light assembly for illuminating the field of view with illumination light from an illumination light source, e.g., one or more light emitting diodes (LEDs), and an imaging lens assembly for capturing return ambient and/or illumination light scattered and/or reflected from the target being imaged over a range of working distances and for projecting the captured light onto the array. Such an imager may include a one- or two-dimensional charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device and associated circuits for producing electronic signals corresponding to a one- or two-dimensional array of pixel information over the field of view.
It is therefore known to use the imager for capturing a monochrome image of the target as, for example, disclosed in U.S. Pat. No. 5,703,349. It is also known to use the imager with multiple buried channels for capturing a full color image of the target as, for example, disclosed in U.S. Pat. No. 4,613,895. It is common to provide a two-dimensional CCD with a 640×480 resolution commonly found in VGA monitors, although other resolution sizes are possible.
As advantageous as both types of electro-optical readers are in reading symbols, it is always desirable to enhance performance. Increasing the intensity or brightness of the laser beam of the laser in the moving laser beam reader will increase the working distance range, because there will be correspondingly more return light to detect from symbols that are further away from the moving laser beam reader. Similarly, increasing the intensity or brightness of the aiming laser in the imaging reader will increase performance, because the aiming pattern will be more visible to an operator, especially for symbols that are further away from the imaging reader.
However, increasing the laser beam intensity too much for either the laser in the moving beam reader or the aiming laser in the imaging reader may violate human eye exposure laser safety standard limits. For example, a class 2 laser is limited to an output power of 1 mW over a base time interval of 250 msec, and a class 1 laser is limited to an output power of 0.39 mW over a base time interval of 10 sec. The laser beam intensity cannot exceed these limits not only in normal operation, but also in the event of reader malfunction or failure of laser power control circuitry specifically provided in each reader to insure that these limits are never surpassed.
The known laser power control circuitry in such readers monitors the laser current in order to provide feedback about the output power of the laser beam. Also, an internal light detector, e.g., a semiconductor monitor photodiode, is typically mounted inside the laser adjacent a semiconductor laser chip, for monitoring the output power of the laser beam. A microprocessor or programmed controller is operatively connected to the monitor photodiode, for controlling a monitored output power of the laser beam by deenergizing the laser when the monitored output power of the laser beam exceeds a safe power level limit.
For example, U.S. Pat. No. 7,609,736 discloses a laser power control arrangement, in which power to such a laser is interrupted upon detection of an over-power condition not conforming to preestablished regulatory standards. During an operational mode, a difference between laser drive currents at two operating points is compared to a difference between laser drive currents at the same two operating points during a calibration mode. A programmed controller sets the operating points by adjusting a digital potentiometer to different potentiometer settings. The over-power condition is recognized when the difference during the operational mode exceeds the difference during the calibration mode by a predetermined amount.
As advantageous as the known laser power control arrangement is in regulating laser output power, performance enhancements in processing speed and accuracy are desirable. For example, the processing software burden on the programmed controller is relatively significant, because the controller, together with an analog-to-digital converter, is tasked with frequently generating different commands to set the potentiometer to different potentiometer settings, measuring the laser drive currents at each setting, and determining and processing differences in such drive currents. In the case of a moving beam reader, these tasks are typically performed once per scan line. In the case of an imaging reader whose controller operates at about 60 frames per second, these tasks are typically performed once per frame, thereby imposing a significant software processing burden. Transitioning between the different potentiometer settings takes a non-negligible transition time, e.g., about 1.5 milliseconds, which some operators may find sluggish. Also, information signals indicative of such measurements are relatively small in magnitude, thereby reducing measurement accuracy. Lasers having higher slope efficiencies than those of currently available lasers could also reduce the magnitude of such information signals. In addition, each time that a low power potentiometer setting is established, the average laser output power and the perceived brightness of the laser light that dwells on the target are reduced for each scan line or frame, which may be objectionable to some operators.
Accordingly, there is a need for easing the software processing burden on the programmed controller, and enhancing the processing speed and increasing the measurement accuracy of such laser power control arrangements in such electro-optical readers.